Hi @Benjamin3 ,
Hi, as per my guess you want to insert an Thumbnail Image (Preview Image) of PDF file into Email, right?
However, as per my knowledge it's not possible, there is an Approach that can help you to insert the Image into Email body.
Prerequisite
1. Image file stored into the SharePoint or publically accessible.
Configurations
Click on advanced Settings and choose "YES" for "is HTML" field, That will allow us to put html content into Email Body.
After that you can insert the Image by just providing the Image tag into the Email Body.
<img src="https://images.pexels.com/photos/433539/pexels-photo-433539.jpeg?auto=compress&cs=tinysrgb&dpr=2&h=650&w=940">
and yes you can also use attibutes into it, and Many more HTML tags too.
Hope this will help you to achive the outcome.
If it works for you please accept this as a solution so other community members will also get benefit of this thread.
Thanks and Regards,
Dev_Nikhil